Beginner: Unknown faces on Mesh
I will walk you through some steps in removing and adding faces with the letter E.
Modeling Resources

Selection Mode
Loop Select
Creating Faces
Deletion Modes



Go into edit mode. Let's first delete all non manifold geometry. Deselect all with A, A. Press ⎈ Ctrl⇧ Shift⎇ AltM to select non manifold geometry. Press X and choose vertices to delete the unnecessary vertices.
Go into face selection mode. ⎈ Ctrl ↹ TabF.
Select the large n-gon which causes the strange face. The select dot show its center. Delete it.
Select the other problematic n-gons. Because some important vertices (marked red) are connected to only the face, deleting the face would be wrong, because these vertices would be deleted as well. Just delete the face with X &gt; Only Faces.
We want to connect the following vertices with an edge, but they're already overlapped by another edge and face.
Hence, delete the faces and the overlapping edges first. 
Now create the remaining face. Select multiple vertices, which you want to connect with a face with ⇧ Shift + Rightclick. Then press F to make an edge / a face.
To select the last edge loop, ⎇ Alt + rightclick on it.